U.S. patent application number 11/279916 was filed with the patent office on 2007-10-18 for method and apparatus for channel estimation.
This patent application is currently assigned to Mediatek Inc.. Invention is credited to Shun-An Yang.
Application Number | 20070242761 11/279916 |
Document ID | / |
Family ID | 38604828 |
Filed Date | 2007-10-18 |
United States Patent
Application |
20070242761 |
Kind Code |
A1 |
Yang; Shun-An |
October 18, 2007 |
METHOD AND APPARATUS FOR CHANNEL ESTIMATION
Abstract
The invention relates to DVB-T system, and in particular, to a
channel estimation method for OFDM symbols. A plurality of symbols
are received to generate a pilot response. A finite impulse
response is generated from the pilot response. A coefficient table
is selected based on the characteristics of the finite impulse
response. The channel is estimated by interpolating the pilot
response based on the coefficient table.
Inventors: |
Yang; Shun-An; (Changhua
County, TW) |
Correspondence
Address: |
THOMAS, KAYDEN, HORSTEMEYER & RISLEY, LLP
100 GALLERIA PARKWAY, NW
STE 1750
ATLANTA
GA
30339-5948
US
|
Assignee: |
Mediatek Inc.
Hsin-Chu
TW
|
Family ID: |
38604828 |
Appl. No.: |
11/279916 |
Filed: |
April 17, 2006 |
Current U.S.
Class: |
375/260 |
Current CPC
Class: |
H04L 5/0048 20130101;
H04L 25/0234 20130101; H04L 27/2647 20130101; H04L 5/0007
20130101 |
Class at
Publication: |
375/260 |
International
Class: |
H04K 1/10 20060101
H04K001/10 |
Claims
1. A channel estimator for an OFDM receiver, comprising: a time
direction interpolator, receiving a plurality of symbols to
generate a pilot response, an IFFT unit, coupled to the time
direction interpolator, generating a finite impulse response from
the pilot response, a DSP, coupled to the IFFT unit, selecting a
coefficient table based on the characteristics of the finite
impulse response; and a frequency direction interpolator, coupled
to the time direction interpolator and the DSP, estimating the
channel by interpolating the pilot response based on the
coefficient table.
2. The channel estimator as claimed in claim 1, wherein: the time
direction interpolator generates the pilot response by collecting
all pilots distributed in the plurality of symbols; and an element
in the pilot response is determined by interpolating the adjacent
pilots in time direction.
3. The channel estimator as claimed in claim 1, wherein the IFFT
unit performs IFFT on the pilot response to generate the finite
impulse response.
4. The channel estimator as claimed in claim 3, wherein: the IFFT
unit filters the finite impulse response by eliminating components
under a predetermined threshold, such that a window is generated,
and the IFFT unit determines a window width and a window position
from the window.
5. The channel estimator as claimed in claim 4, further comprising
a memory, providing a plurality of coefficient tables each
adaptable for a specific window width, wherein the DSP is coupled
to the memory, selecting a coefficient table based on the window
width.
6. The channel estimator as claimed in claim 5, wherein the DSP
generates a plurality of coefficient vectors from the coefficient
table based on the window position.
7. The channel estimator as claimed in claim 6, wherein: the
coefficient tables comprise a plurality set of real numbers; and
the DSP rotates the real numbers by an angle corresponding to the
window position, such that the coefficient vectors are
generated.
8. The channel estimator as claimed in claim 7, wherein the
frequency direction interpolator multiplies the coefficient vectors
with elements in the pilot response to rebuild the channel.
9. A channel estimation method for an OFDM receiver, comprising:
receiving a plurality of symbols to generate a pilot response;
generating a finite impulse response from the pilot response;
selecting a coefficient table based on the characteristics of the
finite impulse response, and estimating the channel by
interpolating the pilot response based on the coefficient
table.
10. The channel estimation method as claimed in claim 9, wherein
the generation of the pilot response comprises: collecting all
pilots distributed in the plurality of symbols; and interpolating
the adjacent pilots in time direction to determine an element in
the pilot response.
11. The channel estimation method as claimed in claim 10, wherein
the generation of the finite impulse response comprises performing
IFFT to the pilot response.
12. The channel estimation method as claimed in claim 11, further
comprising: filtering the finite impulse response by eliminating
components under a predetermined threshold to generate a window;
and determining a window width and a window position from the
window.
13. The channel estimation method as claimed in claim 12, further
comprising providing a plurality of coefficient tables each
adaptable for a specific window width, wherein one coefficient
table is selected based on the window width to estimate the
channel.
14. The channel estimation method as claimed in claim 13, further
comprising generating a plurality of coefficient vectors from the
coefficient table based on the window position.
15. The channel estimation method as claimed in claim 14, wherein:
the coefficient tables comprise a plurality set of real numbers;
and the generation of the coefficient vectors comprises rotating
the real numbers by an angle corresponding to the window position,
such that the coefficient vectors are generated.
16. The channel estimation method as claimed in claim 15, wherein
the pilot response interpolation comprises multiplying the
coefficient vectors by elements in the pilot response to rebuild
the channel.
Description
BACKGROUND
[0001] The invention relates to OFDM communication systems, and in
particular, to a channel estimation method for DVB-T system.
[0002] FIG. 1a shows conventional DVB-T symbols. The horizontal
rows are DVB-T symbols comprising a plurality of sub-carriers,
arranged in vertical time direction. The white nodes frequency
direction interpolator 110 carry data, and the black nodes such as
210a, 210b and 210c, are pilots. The distribution of pilots varies
in time direction, 10 periodically repeating every four symbols.
Conventionally, pilots are utilized to estimate the channel. For
example, a pilot response at the fifth row (t=5) is generated by
interpolation of adjacent pilots in time direction. The pilots 210a
and 230a interpolate the pilot 220a, the pilots 210b and 230b
interpolate the pilot 220b, and the pilots 210c and 230c
interpolate the pilot 220c. The interpolation may be a linear
interpolation, and is also referred to as a time direction
interpolation (TDI).
[0003] FIG. 1b shows a pilot response obtained from the DVB-T
symbols of FIG. 1. The pilots H.sub.0 to H.sub.5 are shown at an
interval. The channels for the sub-carriers therebetween, such as
H.sub.a and H.sub.b, are estimated by frequency direction
interpolation. Conventionally, the frequency direction
interpolation may be accomplished through various algorithms, such
as linear interpolation, second order interpolation, third order
interpolation, bi-linear interpolation and fixed finite impulse
response (FIR) interpolation. The estimated channel is utilized to
restore transmitted data in the sub-carriers in an equalization
process, therefore an efficient channel estimation method can
improve the DVB-T receiver performance.
SUMMARY
[0004] An exemplary channel estimation method for an OFDM receiver
is provided. A plurality of symbols are received to generate a
pilot response. A finite impulse response is generated from the
pilot response. A coefficient table Is selected based on the
characteristics of the finite impulse response. The channel is
estimated by interpolating the pilot response based on the
coefficient table.
[0005] The generation of the pilot response may accomplished by
collecting all pilots distributed in the plurality of symbols, and
interpolating the adjacent pilots in time direction to determine an
element in the pilot response. Generation of the finite impulse
response may accomplished by performing IFFT on the pilot
response.
[0006] The finite impulse response is further filtered to eliminate
components under a predetermined threshold to generate a window,
and a width and a position of which are determined.
[0007] A plurality of coefficient tables are further provided, each
adaptable for a specific window width. One coefficient table is
selected based on the window width to estimate the channel.
[0008] A plurality of coefficient vectors are further generated
from the coefficient table based on the window position. The
coefficient tables comprise a plurality set of real numbers, and
the coefficient vectors are generated by rotating the real numbers
by an angle corresponding to the window position. The coefficient
vectors are multiplied with elements in the pilot response to
rebuild the channel.
[0009] Another embodiment of the invention provides a channel
estimator performing the described method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The following detailed description, given by way of example
and not intended to limit the invention solely to the embodiments
described herein, will best be understood in conjunction with the
accompanying drawings, in which:
[0011] FIG. 1a shows conventional DVB-T symbols;
[0012] FIG. 1b shows a pilot response obtained from the DVB-T
symbols of FIG. 1;
[0013] FIG. 2 is a block diagram of a channel estimator according
to an embodiment of the invention;
[0014] FIG. 3 shows a finite impulse response generated from the
pilot response;
[0015] FIG. 4 shows an embodiment of the frequency direction
interpolation; and
[0016] FIG. 5 is a flowchart of the channel estimation method
according to the invention.
DETAILED DESCRIPTION
[0017] A detailed description of the present invention is provided
in the following.
[0018] FIG. 2 is a block diagram of a channel estimator according
to an embodiment of the invention. A novel interpolation method is
provided. A time direction interpolator 102 receives a plurality of
symbols from a front end such as a FFT unit (not shown), with frame
synchronized and guard interval removed, such that a symbol array
shown as FIG. 1a is obtained, comprising a plurality of consecutive
symbols. The pilots in the symbols are scattered as the
specification defines, distributed periodically to facilitate
channel estimation. The time direction interpolator 102 performs a
time direction interpolation to generate a pilot response
comprising the interpolated pilots as shown in the fifth symbol in
FIG. 1a. A leaky integrator may be utilized to perform the time
direction interpolation, with pilot response an averaged result.
For a DVB-T 2K mode, a symbol may comprise 1705 effective
sub-carriers, 1/3 thereof interpolated to form the pilot response
since the pilots are scattered every 3 sub-carriers. The elements
in the pilot response vary with mode. For example, including the
first pilot, the elements are 569 in the 2K mode, 1137 in the 4K
mode, and 2273 in the 8K mode. The pilot response represents a
preliminary channel, and a frequency direction interpolation is
required to rebuild a complete channel for restoration of every
sub-carrier in the symbols. An IFFT unit 104 is coupled to the time
direction interpolator 102, performing IFFT to generate a finite
impulse response from the pilot response. In the IFFT unit 104, the
finite impulse response is filtered to eliminate components under a
predetermined threshold, such that a window is generated. The
threshold may be a fixed value, or a ratio of the maximum magnitude
in the finite impulse response. The remaining impulses after
filtering form a window, and the window width t.sub.1 and window
position to are determined. The window width t.sub.1 is the
duration from the first impulse to the last impulse, and the window
position t.sub.0 is the time index of the center of the window
width t.sub.1. The t.sub.0 and t.sub.1 are sent to the DSP 108. The
DSP 108 is coupled to the IFFT unit 104 and a memory 106. The
memory 106 provides a plurality of coefficient tables each
adaptable for a specific window width, and the DSP 108 selects one
of the coefficient tables based on the window width t.sub.1 of the
finite impulse response. The coefficient tables are programmable
sets of real numbers specifically designed for the frequency
direction interpolation. After determining a coefficient table
according to the window width, the real numbers therein are rotated
by the DSP 108 with an angle corresponding to the window position
to, thus a plurality of coefficient vectors are generated. In this
way, the coefficient vectors are a function of the window width
t.sub.1 and window position to, and the frequency direction
interpolation can be performed therewith. The frequency direction
interpolator 110 is coupled to the time direction interpolator 102
and the DSP 108, multiplying the coefficient vectors with elements
in the pilot response to rebuild the channel.
[0019] FIG. 3 shows a finite impulse response generated from the
pilot response. As described, the pilot response may comprise
different numbers of elements in different modes, and an IFFT is
performed to generate the finite impulse response. Specifically, to
facilitate the IFFT implementation, the number of elements selected
to perform the IFFT may be an exponent of 2, such as 2.sup.M where
M is an integer. The IFFT may be performed multiple times over a
period to obtain averaged results among a plurality of symbols,
thus the finite impulse response is averaged. An autoregression
moving average algorithm may also be applied to keep the finite
impulse response updated. A threshold V.sub.th is set to filter
noise components. As described, the threshold V.sub.th may be a
fixed value or a ratio of the maximum magnitude in the finite
impulse response. The remaining impulses exceeding the threshold
V.sub.th are deemed to be valid channel paths, and a window 302 is
formed therefrom. The duration from the first impulse to the last
impulse in the window 302, is determined to be the window width
t.sub.1. The time index of the middle of the window is determined
to be the window position t.sub.0. The window width is also
referred to as a channel length, and the window position is the
channel position. One of the coefficient tables is selected based
on the window width, such that the interpolation error in the
frequency direction interpolator 110 can be minimized.
[0020] FIG. 4 shows an embodiment of the frequency direction
interpolation. A plurality of coefficient vectors are provided to
perform the interpolation. For example, three coefficient vectors
are provided as W.sub.a, W.sub.b and W.sub.c: W.sub.a={W.sub.a1,
W.sub.a2, W.sub.a3, W.sub.a4, . . . , W.sub.aN} W.sub.b={W.sub.b1,
W.sub.b2, W.sub.b3, W.sub.b4, . . . , W.sub.bN} W.sub.c={W.sub.c1,
W.sub.c2, W.sub.c3, W.sub.c4, . . . , W.sub.cN}
[0021] where the coefficients therein can be described as:
W.sub.ak=e.sup.j(3M-3k-1).THETA.R.sub.ak k=1 to N
W.sub.bk=e.sup.j(3M-3k).THETA.R.sub.bk k=1 to N
W.sub.ck=e.sup.j(3M-3k+1).THETA.R.sub.ck k=1 to N
[0022] The number N can be 2M+1, where M is an integer not
exceeding 1/2 of the element numbers in the pilot response, and the
angle .THETA. is obtained by the window position t.sub.0: .theta. =
2 .times. .times. .pi. t 0 L ##EQU1##
[0023] wherein L is a predetermined value corresponding to the FFT
size when receiving the symbol. The R.sub.ak, R.sub.bk and R.sub.ck
with k=1 to N, are real numbers provided in the selected
coefficient table. Thus, the coefficients in the coefficient
vectors are obtained by the DSP 108 transforming the real numbers
in the selected coefficient table based on the angle .THETA..
[0024] As shown in FIG. 4, the n-th group of the pilot elements
comprises A.sub.n, B.sub.n and C.sub.n, where B.sub.n is the n-th
pilot and the A.sub.n and C.sub.n are adjacent channels to be
determined. Thus, the channel corresponding to the n-th group of
sub-carriers can be estimated as:
A.sub.n=W.sub.a1B.sub.1+W.sub.a2B.sub.2+W.sub.3B.sub.3+ . . .
+W.sub.aNB.sub.N
B.sub.n=W.sub.b1B.sub.1+W.sub.b2B.sub.2+W.sub.b3B.sub.3+ . . .
+W.sub.bNB.sub.N
C.sub.n=W.sub.c1B.sub.1+W.sub.c2B.sub.2+W.sub.c3B.sub.3+ . . .
+W.sub.cNB.sub.N
[0025] Note that the n-th pilot B.sub.n itself is updated from the
original known value, and the equation can be generalized to the
forms of: A n = k = - M M .times. B n + k .times. W ak ##EQU2## B n
= k = - M M .times. B n + k .times. W bk ##EQU2.2## C n = k = - M M
.times. B n + k .times. W ck ##EQU2.3##
[0026] In this way, the channel corresponding to every sub-carrier
in a symbol can be obtained from the interpolation equation. After
the channel estimation in the frequency direction interpolator 110,
the symbols are sent to an equalizer for further processes.
[0027] FIG. 5 is a flowchart of the channel estimation method
according to the invention. In step 502, a plurality of symbols are
received to generate a pilot response. In step 504, a finite
impulse response is generated from the pilot response. In step 506,
a coefficient table is selected based on the characteristics of the
finite impulse response. In step 508, the channel is estimated by
interpolating the pilot response based on the coefficient
table.
[0028] While the invention has been described by way of example and
in terms of preferred embodiment, it is to be understood that the
invention is not limited thereto. To the contrary, it is intended
to cover various modifications and similar arrangements (as would
be apparent to those skilled in the art). Therefore, the scope of
the appended claims should be accorded the broadest interpretation
so as to encompass all such modifications and similar
arrangements.
* * * * *